1. Field of the Invention
The present invention relates to a method for charging a battery module. More particularly, the present invention relates to a method for charging a smart battery module in multiple stages.
2. Description of Related Art
With increasing performance of a processor, increasing of application programs, and advancing of graphic functions, power requirement of a portable computer is increased accordingly. To cope with the power requirement of the portable computer during normal operation thereof, a plurality of parallelly-connected battery cell sets is generally connected in serial and assembled in a battery module, so as to provide sufficient power for the portable computer.
As battery capacity increases, how to rapidly and safely charge the battery module has become one of the major subjects to various manufacturers. Due to special design of the battery module, the power of the battery module may be varied in different time points or under different charging environment, and therefore a plurality of charging methods are developed based on the above features of the battery module.
A power management circuit disclosed in Taiwan patent No. 250713 is used for controlling charging parameters provided to a battery. FIG. 1 is a block diagram of a conventional power management circuit. Referring to FIG. 1, a power management circuit 100 includes a power control circuit 110, a control signal generating circuit 120 and a current control circuit 130. The power control circuit 110 is used for providing a power control signal representing an output power level of a direct current (DC) power supply, and the control signal generating circuit 120 is used for reducing the charging parameters provided to a battery when the power output level exceeds a predetermined power threshold level. Moreover, the current control circuit 130 is used for providing a current control signal representing a current output level of the DC power supply. The control signal generating circuit 120 may further compare the current control signal to a current threshold signal representing a current threshold level, and when the current output level exceeds the current threshold level, the control signal generating circuit 120 may further reduce the charging parameters provided to the battery. As described above, in the conventional technique, when the power of the battery reaches a current threshold level during battery charging, the power supplied for battery charging is then reduced.
FIG. 2 is a schematic diagram illustrating a charging state of a conventional battery. Referring to FIG. 2, a charging method thereof includes two charging stages, wherein a constant current charging is applied to a first stage (t=0˜t1) thereof, and a charging curve 210 represents variations of voltage VPC of a battery module. When the voltage VPC of the battery module reaches a voltage Vinc provided by a charger, a second charging stage (t=t1˜t2) is started, by which a constant voltage charging is applied, until the battery module is fulfilled (t=t2). According to such method, battery charging is only performed according to an overall voltage of the battery module, and can not be adjusted according to the charging state of each parallelly-connected battery cell set. However, an initial voltage and the charging state of each parallelly-connected battery cell set may be different, and therefore a problem that the battery module is probably still under charging if the voltage of a certain parallelly-connected battery cell set exceeds a safety value (while the overall voltage of the battery module does not exceed the safety value) may be occurred, and such problem not only reduces lifespan of the parallelly-connected battery cell sets, but also leads to a risk of over charging of the battery.
FIG. 3 is a schematic diagram illustrating a charging state of a conventional battery. Referring to FIG. 3, different from the aforementioned method, in the present charging method, the voltage of each parallelly-connected battery cell set in the battery module may be detected, and charging mode of the whole battery module may be adjusted according to a maximum value of the detected voltages. In detail, the constant current charging is still applied to the first charging stage (t=0˜t1), and a curve 310 represents variations of a maximum voltage Vemax detected from the parallelly-connected battery cell sets of the battery module, and a curve 320 represents variations of a minimum voltage Vemin detected from the parallelly-connected battery cell sets of the battery module. When the maximum voltage Vemax of the parallelly-connected battery cell sets reaches a rated voltage Vcoff that the parallelly-connected battery cell sets may bear, the power supplied to a charger thereof is then cut off, and now the maximum voltage Vemax of the parallelly-connected battery cell sets drops accordingly until the maximum voltage Vemax of the parallelly-connected battery cell sets drops to a lower limit value Vcon thereof, and then the power supplied to the charger is restored to increase the maximum voltage Vemax of the parallelly-connected battery cell sets. Again, the power supplied to the charger is cut off when the maximum voltage Vemax of the parallelly-connected battery cell sets reaches the rated voltage Vcoff. The charger is repeatedly turned on/off until all the parallelly-connected battery cell sets are fulfilled. In the second charging stage (t=t1˜t2), the value of the charging current is determined according to the variations of the minimum voltage Vemin detected from the parallelly-connected battery cell sets of the battery module, wherein when the minimum voltage Vemin of the parallelly-connected battery cell set exceeds a voltage Vincc supplied to the parallelly-connected battery cell set by the charger, the second charging stage is started, by which the value of the charging current is gradually adjusted, and the power is also discontinuously supplied to the charger according to the variations of the maximum voltage Vemax of the parallelly-connected battery cell sets until the battery is fulfilled (t=t2). Though the aforesaid method, the problem of over charging on an individual parallelly-connected battery cell set may be solved, it has to take a relatively long time to charge the battery module as the power of the charger is discontinuously supplied. Moreover, frequent charge and discharge of the battery may reduce the lifespan thereof, which is still not an optimal charging method.